Endodontists spend 70-80% of their time treating molar teeth. That is where we live. The experience that is gained by treating (and frequently retreating) literally tens of thousands of molars is part of what differentiates the Generalist from the Specialist. After treating that many teeth, certain principles become self evident. The first of these is an Endodontic cliché: Access is everything. You can't treat what you can't see.
Endodontists spend a LOT of time with their access. Why? Because establishing a "Glide Path" for your instruments not only places less tension on them, it makes file insertions into the orifices easier and therefore actually speeds up the filing sequence. The extra time spent establishing smooth walls, a nice transition to the orifice and good visibility more than makes up for the extra effort. The most common problems I see with endodontic treatment relate to lack of proper access. The access is too small in virtually all of these cases, resulting in missed canals or skinny, ledged and short filled cases, many with broken files embedded in them. One of the most important endodontic concepts in treatment of molars is "limited, purposeful straightening" of the canals that allows your instruments to reach the apex with the minimum of tension on them. This principle applies to all instruments whether they are Ni-Ti or Stainless Steel files, hand or motor driven. And it all starts with the access.
Endodontists HATE it when you send them cases have been "played with" (ie/ Gee, Rob I just put a "couple of files" down the canals, now I can't reach the apex.) Sometimes we can get by these ledges or broken files but frequently we send the case back to you, filled to that same short level that you couldn't negotiate! There is no "magic secret" for unblocking canals like this, when they occur close to the apex. The trick is understanding that when you are not sure of patency, applying apical forces with the tip of a #15 or 20 file - even for just a second or two- is the kiss of death. That is why endodontists go through so many small files in their attempts to maintain patency and are constantly using small (#8, 10 and #15) files to "play tag" with the PDL.
Another of the most frequent problems that I see with mandibular molars is poor negotiation of the MB canal in the "average case". Sometimes clinicians complain that they either cannot initially negotiate the small files to the foramen. Other times they become blocked out once their instruments reach a size #20 or 25. They then struggle to regain the patency that was achieved with the initial working length file only to hit a brick wall. In frustration, the canal is either filled short, ledged or sometimes perforated.
It happens because:
(1) in the hands of most clinicians instruments can only negotiate a maximum of 2 curves at a time and
(2) inadequate glide path has been established during initial access preparation.
Lets examine why this happens and learn how to prevent these errors.
We must understand the anatomy of the mandibular molar in order to know how best to access the MB canal. If we examine a mandibular molar tooth from the mesial aspect we see that the ML orifice is most frequently closely positioned under the height of the ML lingual cusp. However, the MB cusp is entirely different, because it's buccal cusp incline is tilted very far to the lingual. In an attempt to "conserve" tooth structure the "standard access" does NOT extend beyond the MB cusp tip. This is a big mistake. If we use the cusp height as the MB limit of our access prep, the MB canal orifice will still be very far buccal…a long way from the cusp tip. We know that virtually all mesial roots curve distally ( In the diagram below that would be INTO the plane of the paper). Many mesial canals also curve buccally or lingually at the midroot - labeled Curve # 2 . Frequently, canals also curve again close to apex (Curve #3) (most often bucco-lingually or distally but this is not a hard and fast rule.) Therefore, when we make our endodontic access and use the cusp tip as the ML limit of our access prep, we virtually have a straight line shot to the ML orifice if we place the file into the access close to the ML cusp tip. In the conservative access, the glide path is easily established for the ML canal and that's why it is less frequently a problem.
BUT, the MB canal is quite different. If we insert a file with no modification of the standard access, in order to enter the MB orifice the file must first make curve toward the buccal (curve 1 on the diagram below)- "the Extra Curve- our nemesis!"). Should you be able to negotiate the first two curves with a file (the Extra curve - 1 and the orifice/midroot curve- 2), the file will be under so much tension from having to make those initial curves that the final apical curvature (Curve 3- in this case INTO or OUT OF the plane of the paper) will be almost impossible to negotiate. The result is -> Your #08, 10 and maybe #15 files may get to the apex, but as you move up to the #20 and #25, they have no chance. They balk at the final curve and are stuck just before curve 3. You push harder, and BINGO…there's the ledge, blockage or perf and it's game over.
Chances are you'll NEVER reach the apex, especially if you have gone to bigger size files to try to "unblock" the canal. Instead, let's consider how removing part of the MB cusp can reduce this problem. Here's how we do it.The way to prevent this from happening is by using the "MB Slot" preparation. This involves removing all or part of the MB cusp and extending the access with a slot in the MB cusp area. By doing this, we remove the need to negotiate this initial Extra curve 1 and get better straight line access to the midroot. With a bit of "preflaring" (either with small GGs, or Ni-Ti Orifice opening burs - .12 or .10 taper) we can further reduce the tension than the MB file is under as it makes its way down the MB canal. Judicious straightening of this portion of the canal in this manner will eventually allow instruments to freely to negotiate the midroot and apical curves (even if the canal does veer sharply at the apex.) Creation of the buccal slot also allows for easier application of GG burs and vertical condensers that do not now have to bend to accommodate the limits of the initial conservative access.One final advantage of the buccal slot is that it frequently allows for direct vision of the MB canal, reducing the need to use a mirror.
A final word about Conservation of Tooth Structure and Endodontics:
Endodontics treatment is tough business. Should you be considering any endo in your own office, it is important to constantly remember that attempting to "conserve tooth structure " or "make the access through the casting as small as possible" is a recipe for failure. Endodontic treatment (especially in multiple rooted cases) demands that instruments have the best chance to reach the foramen with the least amount of coronal interference . This means that (like it or not) cusps, restorations and even entire castings may have to be removed in order to provide the proper visibility and access. Attempting to "save" parts of teeth, preserve and reuse castings or patch existing restorations to save costs to the patient can only lead to overall compromise of the case, both endodontically and restoratively. It won't matter how small your access is if the patient's periapical pathology continues and the endodontic treatment is not successful. Proper treatment demands that we do it right, the first time….even if it does mean that the restoration must be replaced.